Recently, development of non-aqueous electrolyte batteries, mainly lithium ion batteries, has been aggressively made. Generally, in non-aqueous electrolyte batteries, a microporous film (separator) is provided between positive and negative electrodes. The separator functions to prevent direct contact between the positive and negative electrodes and passes ions through an electrolytic solution held in micropores.
In order to improve cycle characteristics and safety of a non-aqueous electrolyte battery, improvement of a separator has been considered. For example, Patent Literature 1 proposes an adhesive porous film, which is produced by applying a reactive polymer onto a porous film followed by drying, in order to provide a secondary battery having excellent electric discharge characteristics and safety.
Recently, in accordance with reduction in size and thickness of portable appliances, it has been required that electricity storage devices such as a lithium ion secondary battery reduce in size and thickness. In contrast, in order for portable appliances to be able to carry for a long time, increasing capacity of the batteries by improving volume energy density has been attempted.
Then, it is required that a separator is improved in adhesiveness to electrodes in view of not only conventional issues on safety performance, such as characteristics (fuse characteristics) of terminating a battery reaction immediately upon the occurrence of abnormal heating, and performance (short circuit characteristics) of maintaining the shape of the battery even if temperature is elevated to prevent a dangerous situation where a positive-electrode material directly reacts with a negative-electrode material, but also uniformity of charge-discharge current and suppression of lithium dendrite precipitation.
Nonuniform charge-discharge current and precipitation of lithium dendrite rarely occur by improving adhesiveness between a separator and battery electrodes, with the result that the charge-discharge cycle life can be extended.
Under the circumstances, to provide adhesiveness to a separator, an attempt to apply an adhesive polymer to a polyolefin microporous film has been made (see, for example, Patent Literatures 1 and 2).